1. The Field of the Invention
The present invention relates to systems and methods for interleaving optical signals. More specifically, the present invention relates to a thin film interleaver for use in Coarse Wavelength Division Multiplexing (CWDM) optical networks and more particularly to a thin film interleaver that is less sensitive to wavelength drift and temperature variations.
2. Description of the Related Art
One goal of optical fiber networks is to maximize the amount of data or information that can be transmitted through a single fiber. One way of increasing the amount of data traffic on a fiber optic network is by using various types of multiplexing arrangements. One type of multiplexing is based on simultaneously sending data through the same optical fiber using multiple carrier signals or beams. Each of the carrier beams has a different frequency or wavelength than the other carrier beams on a particular fiber. This type of multiplexing is commonly referred to as wavelength division multiplexing (WDM). Two types of WDM systems are CWDM and Dense Wavelength Division Multiplexing (DWDM). In CWDM, for example, signals are sent using lasers with wavelengths that are between 1370 nm and 1610 nm at 20 nm increments.
One of the optical components that is often used in WDM systems is an interleaver. Generally, an interleaver is an optical component that can be used as both a multiplexer and a demultiplexer. When used as a multiplexer, the interleaver can combine the optical signals carried by a pair of optical fibers into a single optical signal on a single optical fiber. For example, if the optical signals being combined each include four separate channels (wavelengths), then the optical signal output by the interleaver will carry eight channels (wavelengths) that are spaced closer together. When used as a demultiplexer, the interleaver separates a single optical signal into a pair of optical signals each carried by different optical fibers. In this case, the channels are more widely spaced.
FIG. 1 illustrates an exemplary interleaver currently available, which is generally designated at 100. The interleaver 100 is manufactured using a fused fiber technique. In this example, two optical fibers are twisted together. Then, the optical fibers are heated at the point where they are connected, the fiber junction 108, causing the fibers to fuse. While still in a heated condition, the twisted and fused fibers are then pulled or stretched to obtain the desired optical characteristics. In the interleaver 100, the fused fiber technique results in a three or four port device that includes, in this example of a three port device, an input fiber 102, an output fiber 104 and an output fiber 106.
The input fiber 102 carries a signal that includes several different channels of various wavelengths. In one example, eight carrier signals or channels are included in the optical signal and are represented as channels λ1, λ2, λ3, λ4, λ5, λ6, λ7 and λ8, where each λn represents a particular carrier, wavelength or channel. Although eight channels are illustrated in this example, other systems may use more or fewer channels. In a CWDM system, for example, the channel designated as λ1 may correspond to the 1470 nm wavelength, the channel designated as λ2 may be the channel spaced at the next 20 nm interval (1490 nm), and so forth.
Returning now to the example in FIG. 1, the multiplexed signal propagates to the fiber junction 108 where the optical fibers have been fused. Because of the way in which the fibers have been fused, the interleaver 100 divides the channels into two separate groups. The first group is represented by the wavelengths λ2, λ4, λ6, and λ8. The second group is represented by the wavelengths λ1, λ3, λ5, and λ7. The first group of wavelengths propagates on the output fiber 104. The second group propagates on the output fiber 106. The output fibers 104 and 106 may be connected to subsequent fused fiber interleavers that further deinterleave the channels. The channels output of the interleaver 100 on the output fibers 104 and 106 are less densely spaced (i.e., 2×channel spacing of 102) than the channels on the input fiber 102.
One of the challenges in modern optical multiplexing systems is addressing the temperature sensitivities of the optical equipment. For example, the DFB lasers that are commonly used in a CWDM system to generate the various channels change the wavelength of their output beam according to the temperature at which the lasers are operating. Because of the sensitivity of the carrier wavelength to temperature, a particular channel may need a bandwidth in some applications that may be, for example, +/−6 nm from the defined carrier channel wavelength. Further, the fused fiber interleaver device itself has some temperature sensitivities. As described previously, the optical characteristics of the fused fiber interleaver are obtained by stretching the glass fibers while they are in a heated condition. Changes in temperature cause the interleaver device to expand or contract, thus changing the optical characteristics of the interleaver.
FIG. 2 shows a graph illustrating the wavelength response of a fused fiber interleaver illustrated in FIG. 1. The wavelength response, illustrated by the curves 210, of a typical fused fiber interleaver is Gaussian. A typical Gaussian response exhibits low loss around the center carrier wavelength. As the channel wavelength drifts away from the carrier wavelength, the response quickly drops off, resulting in higher signal loss. In other words, wavelength drift can result is significant signal loss. Generally, small frequency shifts and corresponding small signal power losses are tolerable. Heavy losses of signal power are less tolerable, as they can reduce the distances over which the optical network can be deployed and can increase error rates.
In addition to wavelength drift and temperature variations, frequency dependent loss and cross talk are other examples of component characteristics that have an impact on the performance of optical components. Interleavers are often integrated into other optical components such as optical add/drop modules (OADM) and interleavers therefore have an impact on the performance characteristics of those optical components.
One method of reducing power loss of a fused-fiber interleaver that exhibits a Gaussian top frequency response is to widen the Gaussian response to provide a wider frequency response. A drawback from having a wider frequency response is that adjacent channels become less isolated as the actual frequency varies from the defined channel frequency. This breakdown of isolation is sometimes referred to as cross-talk. Cross-talk results in data from one channel being mixed into adjacent channels thus making the data on the adjacent channel more difficult or impossible to extract.
To reduce cross talk problems, the frequency response is shaped so that the losses are high as a particular frequency on a channel drifts towards other channels. Adjusting either the frequency dependent loss or cross talk performance characteristics has an adverse affect on the other.